1. Field of the Invention
The present invention relates to a thermal recording apparatus which performs recording pixels each having a predetermined density by selectively driving a plurality of heating-elements mounted on a thermal head.
More particularly, the invention relates to a thermal recording apparatus, in which a pixel is formed by a plurality of dots arranged in a matrix form, capable of greatly increasing the number of gradation levels while maintaining resolution and providing a smooth change in the gradation of each pixel by forming a dot pattern in accordance with the energy amount of a dot formed in the matrix, the energy amount being quantized in multi levels.
2. Description of Related Art
Conventionally, there have been proposed various thermal recording apparatus using a thermal head for printing images with multi gradation levels.
For example, Japanese Patent (JP) publication No. 6(1994)-30887 discloses a thermal printer which selectively causing a plurality of heating resistance elements mounted on a thermal head to generate heat for printing out images is provided with means for driving the resistance elements on an every second pixel basis, which can produce an even density distribution in the printed images. The printer is also provided with means for energizing the resistance elements associated with odd pixels and those associated with even pixels at different time points, yet a plurality of resistance elements out of each of the odd and even resistance elements at the same time.
JP publication No. 7(1995)-46828 discloses a thermal transfer type recording apparatus including a thermal head provided with a plurality of heating-elements arranged in line, the thermal head being moved relative to a substantially perpendicular direction with respect to the arrangement direction of the heating-elements, yet the heating-elements being selectively caused to generate heat for transferring an ink to a desired position on printing paper, whereby to provide a gradation of each pixel in accordance with a dot matrix region of ink transferred. This apparatus further includes a memory which stores in advance plural dot patterns formed by dots in the matrix form to be transferred and the energy supply level of each heating-element, the energy supply level being quantized to multi levels, a determining means which determines, per pixel, the dot pattern constituted of dots in a matrix assigned to the pixel and the energy supply level of each heating-element corresponding to each dot based on the density for the multi-gradation level signal, referring to the memory, and means for causing the generation of heat and the movement of the thermal head in accordance with the pattern and energy supply level of the pixel, determined by the determining means. The memory stores a pattern of dots arranged in the relative movement of the thermal head as a pattern formed by a plurality a dots to be transferred in a matrix. The memory stores, preferably, the dot patterns in a low, middle, and high density ranges respectively, in which the pattern in a low density range comprises a single dot to be printed, the pattern in the middle density range comprises dots arranged in the direction of the relative movement of the thermal head, and the pattern in the high density range comprises at least one dot in addition to the dots arranged as above. The apparatus uses both a pseudo half tone printing method for printing images with gradation or gray levels and a heat energy control method. The apparatus also uses the patterns of dots arranged in the direction of the relative movement of the thermal head to printing paper for a pattern comprising a plurality of dots to be printed in a matrix to provide a gradation level. Accordingly, the apparatus can print half tone images improved in resolution and image quality.
However, in the thermal printer disclosed in JP publication No. 6-30887, one pixel is constituted of a plurality of dots and the heating-element corresponding to each dot is driven to control the gradation level of each pixel, so that the resolution and the number of gradation levels are degraded if one pixel is formed by a small number of dots.
In the thermal transfer recording apparatus disclosed in JP publication No. 7-46828, the pattern is formed by a part of dots in a matrix of each pixel. As a result, there occur problems that there is a limitation in a minimum number of dots forming one pixel and that the constant matrix size can not provide the large increment of the number of gradation levels without degrading resolution.